1. Field of the Invention
The present invention relates to a method of preparing cyclic formals which are useful as solvents, intermediates of drugs, detergents, starting materials for resins, and like substances. More particularly, it relates to a method of preparing cyclic formals which contain only small amounts of by-product impurities or unreacted starting materials.
2. Description of the Related Art
Cyclic formals typified by 1,3-dioxolan, 1,4-butanediol formal, diethylene glycol formal, 4-methyl-1,3-dioxolan, 1,3-dioxan and 1,3,5-trioxepan are known to be obtainable from a cyclizing reaction between a corresponding glycol and an aldehyde, and between a corresponding alkylene oxide and an aldehyde. For example, German patent No. 1914209 discloses a process for preparing a typical cyclic formal, 1,3-dioxolan, by reacting glycol with formaldehyde in the presence of an acid catalyst, and Ind. Eng. Chem., 46,787(1954) and U.S. Pat. No. 3,857,759 both disclose a process for preparing 1,3-dioxolan by reacting glycol and paraformaldehyde in the presence of an acid catalyst.
However, research conducted by the inventors of the present invention on a method of preparing cyclic formals starting from a glycol and an aldehyde revealed that considerable amounts by-product impurities and unreacted starting materials, especially formaldehyde, were contained in the distillate obtained when an ordinary reaction vessel of an evaporator type was used for the reaction and the vapor produced was taken out continuously. The presence of by-product impurities or unreacted starting materials in a distillate should be avoided if possible since it greatly affects the subsequent purification process of cyclic formals and renders the purification process cumbersome and complicated.